You would likely only need the one Topargee gauge, installing it after the tank junction. With one Topargee, your first two complete tank usages will quickly tell you exactly how much water youll get out of each tank just by taking note of the gauge reading when it starts sucking air on one tank before flicking over the other. If you isolate each tank with tap one Topargee should do everything you require.
Your kind of right about the Drifter, its certainly not accurate enough in itself to rely on, but once our Drifter starts to flash on each tank I know I have around 25ltrs left in each, which for us is one day. That's good enough for me, Id prefer to stick my head under side of the van anyway and have a look at the clear pipe, and keep my $129 in my pocket. From experience I know I can get 10 days out of our water supplies, so only if I intend to stay off the grid in excess of 10 days that I need to bother managing it anyway.